Heretofore, dual chamber orifice fitting devices have been used for measuring the flow of fluids through pipe while utilizing an orifice plate in the flow line. An orifice plate consists of a flat plate with a predetermined opening therein. By measuring the pressure differential on opposite sides of the orifice plate, the rate of fluid flow can be determined in the pipeline.
An orifice plate must be maintained in relatively good condition to assure measurement accuracy. The opening or bore of the plate must remain clean and its upstream edge must remain sharp and free of nicks. Debris or foreign matter built up on the faces of the plate can also adversely affect the accuracy of the flow measurement. Since the fluid flowing in pipelines may contain substances or particulates which may adversely affect the orifice plate, it is desired that the plate be periodically inspected. In fact, it is common practice in the industry and even recommended that the plate be inspected on a periodic basis, such as each month. This inspection involves removing the orifice plate from the pipeline, inspecting it, and either reinstalling it in the pipeline or, should its condition require, replacing the orifice plate. Also, in the event flow conditions change, such as an increase or decrease in the fluid flow rate, an orifice plate with a larger or smaller opening or bore may be required. This would require removing the old orifice plate and replacing it with a new one with a different bore. In either of the aforementioned situations, it is necessary to provide a means for the removal and installation of the orifice plate in the orifice fitting or flow measurement device.
It is desirable that the orifice plate be removed and replaced without shutting down the pipeline so that flow through the pipeline is maintained. For this purpose, a dual chamber orifice fitting device has been provided in which an inner or lower chamber is provided in continuous fluid communication with the flow line while an outer or upper chamber is isolated for removal of the orifice plate. A valve member or isolation member is positioned between the chambers and when closed to separate the chambers, the outer chamber may be exposed to atmosphere. However, the lower chamber does not have to be reduced to atmospheric conditions to remove the orifice plate from the pipeline. Thus, the flow of fluid does not have to be stopped.
One of the most common designs for a dual chamber orifice fitting device is a "Senior Orifice Fitting" manufactured and sold by Daniel Industries, Inc. of Houston, Tex. and described in Catalog 100A, entitled "Senior Orifice Fitting Technical Manual", published by Daniel Industries, Inc. This design is also disclosed in U.S. Pat. Nos. 1,996,192 and 2,688,987, which are incorporated by this reference.
In most dual chamber orifice fitting devices, an operator must rotate at least two, and possibly three, shafts to effect removal and replacement of an orifice plate. The shafts, in turn, rotate gears that provide a means for raising or lowering the orifice plate between the chambers, or a means for opening and closing an isolation valve between the chambers that will isolate the upper chamber from line pressure. In addition to rotating these shafts, the operator must rotate other devices, such as a bleed valve that can reduce the internal pressure of the outer chamber to atmospheric conditions, a valve that can equalize the pressure between the upper and lower chambers, or bolting to open the upper chamber. Thus, many steps are involved in the removal and installation of an orifice plate. To facilitate the process, some manufacturers have standardized on the wrench required to operate the shafts, valves, and bolting. An operator may use the same wrench in each of these steps while moving the same wrench from various shafts, valves, and bolts, or the operator may choose to use more than one wrench, but in all situations there are many steps required to remove and install an orifice plate and an operator must turn a number of items in the proper sequence.
The relatively large number of steps to remove and install an orifice plate can be confusing to an operator. To assist the operator in operating the device, manufacturers place instruction plates on the orifice fitting devices to help guide the operator through the procedure in the proper sequence. However, due to paint or a build up of foreign matter on the instruction plate, or through natural weathering, the instruction plate is often difficult or impossible to read. In addition, instruction plates, name plates and the like, are often removed from valves and orifice fitting devices prior to painting and are not always repositioned after the painting. Thus, the operator operating the orifice fitting device is often without instructions readily available, a circumstance that can add to a possibly confusing situation. Even if the instruction plate is present and readable, an operator may not read it as each step is performed due to the familiarity of the operator with the procedure. An operator may also momentarily forget the proper sequence of steps or become confused. In addition, most manufacturers standardize on a common side of the orifice fitting device on which the shafts are exposed for operation. Sometimes the operating shafts are located on a side other than the normal side to facilitate the positioning of an orifice fitting device within a limited space. This alternate placement of the operating shafts requires that the shafts be rotated differently with regard to previously known procedures and this can also cause confusion.
In all of the above mentioned designs or orifice fitting devices, a dangerous condition may occur when the internal pressure of the upper chamber is reduced to atmospheric conditions and the upper chamber itself is opened for removal of the orifice plate. When this condition occurs, the valve or device that isolates or separates the upper and lower chambers is the only member that prevents the line pressure from rapidly exhausting through the outer opening of the upper chamber. The operator operating the orifice fitting device must be careful not to open the isolation valve between the inner and outer chambers. If the isolation valve is opened when the upper chamber is opened to atmosphere, highly pressurized fluid from the main flow line will rapidly vent through the rather large opening between the upper and lower chambers and out through the upper opening of the upper chamber into the atmosphere. Obviously, this is not a safe condition for the operator operating the orifice fitting device as the operator is usually in very close proximity to the opening to atmosphere. Further, when reinstalling an orifice plate in many designs of the orifice fitting device, a relatively large metal plate or carrier for the circular orifice plate is placed within the upper chamber. An operator must proceed with a number of steps to close the upper chamber, isolate the upper chamber from atmosphere, and repressurize the upper chamber. However, as mentioned before, the operator may become confused and forget to take the steps in sequence and may inadvertently open the isolation valve between the chambers before isolating the upper chamber from atmosphere. As soon as the isolation valve between the chambers is opened, the rapidly escaping pressurized fluid rapidly forces any unsecured member, such as a carrier device, out of the upper chamber resulting in possible damage to property or injury to personnel.
Because of the complexity of operating such a dual chamber orifice fitting device and the resulting possibility that an operator may perform an operating step out of the proper sequence, particularly opening the isolation valve between the dual chambers out of sequence, there is a need for a safety device or interlock that will effectively prevent an operator from manually opening an isolation valve between the dual chambers when a pressure differential exists in the two chambers.